This invention relates generally to the navigation of spacecraft and, more specifically, to systems for determining the attitude, position and orbit of a spacecraft autonomously, i.e., independently of externally supplied data. Autonomous navigation of earth-orbiting satellites has been of interest since the early 1960s. Various complements of instruments have been used, including horizon scanners, landmark trackers, star sensors, gyros, and space sextants. Recent activities in the field have used earth horizon sensing to determine altitude, crosslinks with other satellites to provide data, and the use of GPS (Global Positioning System) to obtain position and orbit data from multiple satellites.
Horizon sensing has in the past been used in conjunction with ground-supplied orbit information. A system known as space sextant used observations of the moon and sun to determine position. This approach requires an extremely high precision of measurement and is, of course, subject to difficulties during periods of poor moon visibility. Systems using GPS or satellite crosslinks are not strictly autonomous. GPS satellites are continually maintained by ground stations, and any satellites used in a crosslink arrangement must also be maintained or monitored from the ground. Direct sensing of the earth's horizon remains as the only low-cost means for implementing autonomous navigation. By itself, however, horizon sensing determines only the altitude of a spacecraft above the earth. Without additional information supplied externally, the spacecraft has not been able to completely determine its position and attitude.
Satellites are used in low earth orbit (LEO) and geosynchronous earth orbit (GEO) for a wide variety of applications, including data, voice and video communication, weather observation, and ecological studies, as well as various military applications. If satellites could be made totally autonomous, there would be a large cost saving because of the elimination of ground support stations. In addition, ground supported satellites are subject to ongoing maintenance costs, and especially in a military context, are prone to serious malfunction if ground support stations are damaged or if control signal transmissions are interfered with.
It will be appreciated, therefore, that there is a need for a totally autonomous spacecraft navigation system. The present invention satisfies this need.